A Slight Twist Gives Interesting Effects on Two Layers

Innovative ultra-thin nanomaterial featuring outstanding qualities. Amazing physical events can occur, for example, when atomically thin layers of individual crystals are stacked vertically. Superconductivity, for example, can be present in a double layer of the magic material graphene that has been twisted by a magic angle of 1.1°.

Physicists Anvar Baimuratov (left) and Alexander Högele in their laboratory at LMU. Image Credit: Jan Greune/LMU

In addition, bilayer semiconductor heterostructures, held together by weak van der Waals forces and made of so-called transition metal dichalcogenides, are the focus of investigation.

Such unique heterostructures, which do not exist in nature, were studied by a research team led by Alexander Högele.

The combination of materials, the number of layers, and their relative orientation give rise to all kinds of new phenomena. In the lab, we can adapt physical phenomena for various applications in electronics, photonics, or quantum technology with properties unknown in natural crystals..

Alexander Högele, Physicist, Ludwig Maximilian University Munich

But like a recent study published in the journal Natural Nanotechnology point out, it is not always easy to understand the events observed during an experiment.

The Moiré Effect Depends on the Layer Orientation

Högele’s team studied a heterobilayer system made of monolayer semiconductor tungsten diselenide (WSe2) and molybdenum selenide (MoSe2) which are held together by van der Waals forces. The appearance of the moiré effect can vary depending on how the various layers are oriented.

This effect—which is very familiar from everyday life—occurs at the nanoscale when two different atomic lattices are stacked on top of each other or when two identical lattices are twisted against each other. The fact that it’s not an optical effect sets the nano case apart.

According to Högele, moiré interference has a significant effect on the characteristics of composite systems and also on tightly bound electron and electron-hole pairs, or excitons, in the quantum mechanical realm of atom-thin crystal heterostructures.

Högele added, “Our work demonstrates that naïve notion of perfect moiré patterns in MoSe heterobilayers2-WSe2 not always applicable, especially for small rotation angles. Therefore, the phenomenological interpretation observed to date has to be partially revised.

There are laterally extended areas that lack moiré disturbances displacing periodic moiré patterns. In addition, there are regions with interesting quantum mechanical phenomena such as quasi-zero-dimensional quantum dots or one-dimensional quantum wires that could be suitable for use in quantum communications because they are based on spatially localized excitons with single-photon emission characteristics.

It is believed that the ideal moiré pattern can be turned into a periodic pattern with triangular or hexagonal tiles in the latter scenario.

Collective Phenomena in Synthetic Crystals

The reason for this seems to be the elastic deformation of the lattice structure which depends on the orientation of the layers.

The atoms are displaced from their equilibrium positions, which results in higher strain in the individual layers but increases the adhesion between the layers. As a result, heterobilayer systems have an energy landscape that can be created and potentially exploited through careful design.

We also observe a collective phenomenon in synthetic crystals, where periodic moiré patterns have a dramatic effect on the motion of electrons as well as their mutual interactions.said Högele.

The understanding of excitons—electron-hole pairs—that are typical for different types of atomic registers in bilayer crystal heterostructures and potentially used in prospective opto-electronic applications is most relevant.

Through the process of absorption of light, this exciton is produced in semiconductor transition metal dichalcogenides and then turned back into light.

The exciton thus acts as a mediator of light-matter interactions in a semiconductor crystalsaid Högele further.

As this study shows, different excitons can form when the heterobilayer system is aligned in parallel or antiparallel, depending on the actual structure.

Högele concludes, “We wanted to study how to fabricate van der Waals heterostructures with properties adapted in a deterministic approach to control the rich emerging phenomenology of correlated effects such as magnetism or superconductivity.

Journal Reference

Zhao, S., et al. (2023) Delight in mesoscopically reconstructed moiré heterostructures. Natural Nanotechnology. doi:10.1038/s41565-023-01356-9.


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